The present invention relates to a slab geometry solid state laser medium, and more particularly, to such a material that has concave edge surfaces to reduce parasitic oscillations.
The rectangular cross-section, slab geometry, face-pumped laser (FPL), such as shown in U.S. Pat. No. 3,633,126, has an optically pumped laser material. Such a material 10 is shown in the present FIG. 1(a), wherein the pump energy is applied to at least one of the pump faces 12a an 12b by flash tubes (not shown). The ends 14a and 14b comprise edge faces. However, such a laser material 10 is prone to internal parasitic oscillations caused by total internal reflections(TIR) in a closed-loop light path 16a from the polished surfaces 12 and 14 of the laser material 10, wherein the direction of the light propagation is indicated by the arrows. Alternate possible parasitic mode paths 16b and 16c are shown in FIGS. 1(b) and 1(c), respectively. Still other modes are possible. In particular, while transverse modes are shown in FIG. 1, longitudinal modes also exist. This cycling parasitic energy depletes the population inversion of the laser material, thus reducing the overall efficiency of the device. At best, the energy in the parasitic oscillations limits the drilling and cutting performance of the FPL when used in such applications by limiting the energy output per pulse and, therefore, the average output power. In the worst case however, the parasitic oscillations can reach dangerously high levels leading to significant material damage.
One way to reduce or eliminate parasitic oscillations in low power slab laser materials is to leave the edge faces 14 of the slab in a rough ground condition typically 600 grit, as is known from the articles "Parasitic Oscillations and Amplified Spontaneous Emission in Face-Pumped Total Internal Reflection Lasers," D.C. Brown et al., SPIE, Vol. 736, pp. 74-83, 1987, and "Parasitic Oscillations, Absorption, Stored Energy Density and Heat Density in Active-Mirror and Disk Amplifiers," D. C. Brown et al., Applied Optics, Jan. 15, 1978, Volume 17, No. 2, pp. 211-224. This is effective in reducing parasitic oscillations since the closed-loop TIR paths 16 are reduced since rays striking the ground edges 14 of the slap 10 are mostly scattered and refracted out of the material instead of being totally internally reflected. However, the ground finish leaves many damage sites, which have been shown to decrease the strength of the material. During operation, these damage sites may lead to significant material failure (cracking). Also in high power slab lasers, the rough ground edges 14 can contribute considerably to the distortion of the optically pumped material, thereby degrading the laser's beam quality.
In order to reduce the negative effects of ground edges 14, laser materials can be chemically etched prior to final polishing. The edge faces 14 are left in the etched condition when the remainder of the slab is polished. The etching increases the strength of the material by leaving a smoother, more damage-free material than do ground edges. Although the etched edge slab suppresses parasitic oscillations better than a polished edge slab, it does not suppress parasitic oscillations nearly as well as the ground edge slab.
Said articles also disclose the concept of beveling or canting the edge faces to reduce parasitic oscillations. However, since such bevels must extend to the ends of the slab, a laser incorporating such a slab would be difficult to seal. Also, since the pump faces each would have a different dimension, the slab is asymmetrical, which would result in asymmetrical thermo-optic distortion of the slab and asymmetrical surface deformation.
It is therefore an object of the present invention to reduce parasitic oscillations in a slab geometry laser material without causing cracking thereof or a reduction in laser performance, such as beam quality, efficiency, etc.